Fatty substance esters are currently used in many applications as diesel fuels, furnace fuel oils, ecological solvents, base compounds for preparing fatty alcohol sulfonates, amides, ester dimers, etc.
They can be obtained from a transesterification reaction carried out according to path I below and optionally a coupled esterification and transesterification reaction, esterification being achieved according to path II below.
Path I:
1 triglyceride+3 alcohols→3 fatty substance esters+glycerin
Path II:
Fatty acid+alcohol→fatty acid esters+water
Fatty acid+glycerin→glyceride+water
In the case of diesel fuel, which is today a major application for fatty substance esters, a certain number of specifications have been established, whose list, limits and methods belong to standard EN 14214 (2003) currently applicable in Europe. The ester must contain at least 96.5 mass % esters, at most 0.8 mass % monoglycerides, at most 0.2 mass % diglycerides and at most 0.2 mass % triglycerides, few free fatty acids (<0.5 mg KOH per g), which may be corrosive, less than 0.25 mass % bonded and free glycerin, and metals only as traces. This requires a precise protocol in order to obtain the desired purity.
When preparing an ester from oil or fat and monoalcohol, depending on the nature of the oil initially used, 10 to 15 mass % of a secondary product, which is glycerin, automatically forms. This glycerin is sold at a high price for various uses, but only if it has high purity. This is obtained after deep purifications in specialized vacuum distillation units.
In short, most commercial ester preparation methods lead quite readily to raw products (esters and glycerin) that must however be deeply purified using various treatments that eventually burden the conversion cost.
It is well known to prepare methyl esters using conventional means such as homogeneous catalysis with soluble catalysts, like soda or sodium methylate, by reacting a neutral oil and an alcohol such as methanol (for example JAOCS 61, 343-348 (1984)). A pure product that can be used as fuel and a glycerin meeting specifications are however obtained only after many stages. Indeed, the glycerin obtained is polluted by alkaline salts or alcoholates, so that the glycerin purification facility is almost as costly as the ester production facility.
Heterogeneous catalysis methods afford the advantage of producing catalyst-free esters and glycerin that are therefore easily purified. It is however often difficult to economically obtain both an ester and a glycerin of high purity.
In order to improve the cost-effectiveness of this method and to limit its operating costs, many teams have taken an interest in the development of new and more active catalysts.
Basic solids have a high catalytic activity for the vegetable oil transesterification reaction, but they may be sensitive to the presence of free fatty acids contained in the feed. This problem can notably be encountered when the oils used in the method are economically interesting, such as, for example, jatropha oil or used oils. Various oxides and mixed oxides, possibly doped, have thus been tested, as described for example in the article by W. M. Antunes et al. (Catalysis Today, 133-135 (2008) 548-554), which describes the transesterification of soybean oil with methanol, using basic solid catalysts such as ZnO, MgO, Al2O3, or mixed oxides derived from hydrotalcites (Mg/Al and Zn/Mg/Al). However, these basic solids have a tendency to deactivate or they are not stable in the presence of too large an amount of fatty acids contained in unrefined or poorly refined oils.
Patent FR-B-2,752,242 filed in the name of the applicant describes the use of solid non-soluble catalysts formed from alumina and zinc oxide or zinc aluminate.
Using acid catalysts allows to work with feeds containing a certain proportion of free fatty acids, which are economically more interesting. An example thereof is the Fe-Zn cyanide solid used by P. Ratnasamy (Applied Catal. A: Gen, 314, 2006, 148) that allows transesterification of vegetable oils, acid or not, with yields close to those obtained with basic catalysts. However, the reaction selectivity, characterized by the formation of ethers, can be degraded by the presence on these solids of Brönsted acid sites bound to protons.
While trying to improve the performances of the transesterification reaction of fatty substances of vegetable or animal origin, the inventors have developed a method using an acido-basic heterogeneous catalyst based on nitrogen-containing metallophosphates having both basic and acid sites.